The prior art ball valve has a body; inner side of the body is formed as an approximate round chamber therein and a through hole communicated to the chamber. A top of the body is formed with an upper penetrating hole and a bottom thereof is a lower penetrating hole. A stem passes through the upper penetrating hole to the chamber. A valve unit is installed to the inner side of body. The opening and closing of the valve unit will control the flow to pass through the valve unit. The valve unit is a ball valve or a half-ball valve. The valve unit has an axial hole for receiving the stem. A cover has a size corresponding to that of the through hole. One side of the cover is formed with a plurality of screw holes so that the cover can be screwed to outer side of the through hole by using screws to lock the cover and the body.
However the prior art ball valve or half-ball valve is easy to make the annular seat or the stem to be destroyed or inclined, and thus the valve will over-used so as to induce drainage of flow at two sides of the valve unit.